Orientation of ferromagnetic particles



Dec. 4, 1962 E. F. R. A. SCHLOEMANN ETAL 3,066,355 ORIENTATION OF FERROMAGNETIC PARTICLES Filed May 29, 1959 36 MOTOR 1 CONTROL 2g VOLTAGE F/G! 3" 20 2/ 5' SOURCE OF PULSE EXCITATION SLI 30 32 33 APPLIED MAGNETIC FIELD 1ST 2ND 3RD 4TH 1ST 2ND AXIS AXIS AXIS AXIS AXIS AXIS FIG. 4

INVENTORS ERNST E R. A. SGHLOEMAN/V MARSHALL H. S/RVE 7' Z d MM ATTORNEY United States Patent Ofifice 3,066,355 Patented Dec. 4, 1962 3,966,355 ORIENTATION OF FERROMAGNETIC PARTICLES Ernst F. R. A. Schloemann, Newton, and Marshall H. Sirvetz, Boston, Mass, assignors to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed May 29, 1959, Ser. No. 816,985 11 Claims. (Cl. 1848) This invention relates generally to ferromagnetic materials and, more particularly, to a method of orienting ferromagnetic particles by means of magnetic field pulses.

Ferromagnetic materials have been used extensively in many microwave devices and, hence, are assuming great importance in microwave systems. It is well known to those skilled in the art that, in ferromagnetic materials having crystalline structures, the magnetization tends to be directed along certain definite crystallographic axes called easy axes of magnetization. The directions of these easy axes of magnetization depend upon the anisotropy constants of the material.

It has been found that some ferromagnetic microwave devices, such as isolators, provide better operation if the easy axes of magnetization of each of the crystals which make up the ferromagnetic material are aligned parallel with each other. Alignment of these axes provides a material with a narrower resonance line width and this usually represents an advantage in microwave system applications.

Despite the fact that it is desirable to obtain ferromagnetic materials all of whose axes are aligned, the ferromagnetic materials that have been used heretofore in microwave devices have generally been composed of randomly directed crystals, the easy axes of magnetization of which are not aligned. This is attributable to the fact that there has been no satisfactory method for producing such an orientation except in very limited cases of one dimensional alignment along only a single axis. This invention, however, provides a method for orienting ferromagnetic particles in more than one dimension so that all the appropriate easy axes of magnetization of the crystals are aligned parallel to each other.

The invention represents an improvement of the method described in copending application No. 820,788, filed on June 16, 1959 by Luther A. Davis, Jr. and assigned to the same assignee as this application. As described in the method of the copending application, the ferromagnetic particles are suspended in an appropriate medium which is subjected to the continuous application of a magnetic field. The suspended particles are rotated in discreet steps so that, during each revolution, the magnetic field is applied along the different directions corresponding to the relative directions of the easy axes of magnetization of the ferromagnetic crystals.

The method thus described, however, is subject to some disadvantages in that the yield stress of the suspending medium, which increases during the process, prevents the attainment of a high degree of orientation. In addition, the process therein described is limited with regard to the types of suspension media that can be used. Moreover, it has been found that the use of a continuously applied magnetic field reduces the flexibility of the process with respect to the time periods involved in establishing the orientation of the material.

The method of this invention represents an improvement over that described in the copending application because it greatly increases the degree of orientation which can be obtained. The process of this invention is quite flexible in that a greater variety of suspension media are available for use and fewer limitations are placed on the time duration of the process.

According to the method of the invention, a container with the ferromagnetic particles is positioned so as to occupy a volume to which suitable magnetic fields may be applied. The particles are supported in a state which initially allows freedom of motion of the crystals. Short pulses of magnetic field are then applied in such a way that, for a particle with the desired orientation, the magnetic field points along the directions of the various easy axes of magnetization in turn. This sequential application of the magnetic field in the form of short pulses is repeated a large number of times in a cyclic manner. Throughout the cyclic process, the freedom of motion of the crystals is gradually restrained so that, at the end of the process, the crystals of the material are substantially prevented from further motion and are essentially frozen in their correctly oriented state.

The invention may be more easily described with the help of the drawing wherein:

FIG. 1 shows the directions of the easy axes of magnetization of a cubic single crystal having a negative anisotropy constant;

FIG. 2 shows the directions of the easy axes of mag netization of a cubic single crystal having a positive anisotropy constant;

FIG. 3 shows a particular embodiment of a means which can be utilized in the method of the invention for orienting a material with a cubic crystalline structure having a negative anisotropy constant; and

FIG. 4 shows a graph of the cyclic waveform of the applied magnetic field pulses as a function of time.

For a material composed of cubic crystals having a negative anisotropy constant, the easy axes of magnetization lie in the directions of the four body diagonals of the cube. There is shown in FIG. 1 a cubic crystalline structure 10 of a ferromagnetic material having a negative anisotropy constant which has its easy axes of magnetization along the diagonals 11, 12, 13, and 14. Nickel ferrite and manganese ferrite are examples of materials of this type.

In FIG. 2 there is shown a cubic crystalline structure 15 of ferromagnetic material having a positive anisotropy constant. For such a material, the easy axes of magnetization lie in a direction parallel to the three cube edges 16, 17, and 18. Ferrites containing appreciable amounts of cobalt are examples of materials of this type.

In one particular embodiment of the method of the invention, the orientation of crystal particles having a negative anisotropy constant, as in the cube of FIG. 1, is accomplished by suspending the particles in a particular medium, such as a liquid having the properties of a viscous fluid. A magnetic field is applied along a first direction corresponding to the direction of one of the easy axes of magnetization. The magnetic field is applied in the form of a very short pulse, the length of which, in one embodiment, is approximately ten microseconds. At the beginning of each pulse for a time period of approximately one tenth of a microsecond, the magnetization vector of each particle is pulled into the direction of the magnetic field. Throughout the remainder of the pulse, a torque is exerted on the particle and this torque tends to rotate the particle in such a way that the nearest direction of easy magnetization is brought into coincidence with the direction of the applied field.

After the pulse has been applied along a first direction corresponding to a first easy axis of magnetization as, for instance, along a direction corresponding to body diagonal 11 in FiG. l, the direction of the applied magnetic field is changed to coincide with a second direction whose geometric relationship to the first direction corresponds to the geometric relationship between a second body diagonal (as, for instance, body diagonal 12) and first body diagonal 11 in FIG. 1. The magnetic field is then applied a second time along this second direction by means of a short pulse in a manner similar to that described for the first diagonal direction. In a similar manner, a short pulse of magnetic field is applied sequentially along third and fourth directions whose geometric relationships to the first and second directions correspond to the geometric relationships that exist between body diagonals 13 and 14 and body diagonals 11 and 12. Thissequential application of the magnetic field pulse along each axis in turn is repeated in a cyclic manner. The wave form of the applied cyclic pulse of magnetic field is substantially equivalent to that shown in FIG. 4 where pulses 30, 31, 32, and 33 represent pulses along the four directions corresponding to the relative directions of the four easy axes of magnetization. As can be seen in that figure, the pulses are repeated in a cyclic manner.

If all of the particles were approximately spherical, there would be a substantially complete alignment of the crystal axes of the particles. However, since the general shape of most particles is nonspherical, complete orientation may not be possible. The detrimental effects of a nonspherical shape of the particles can be minimized by making the pulse length sufficiently short. Thus, the pulse length is made shorter than the time required by a representative particle to rotate to its oriented position. Under these conditions, the torques produced by the anisotropic characteristics of the crystal act in the same direction for each of the repeated applications of the pulse field whereas the torques produced by the nonspherical shape of the material tend to average out.

If the suspension medium behaves like a viscous fluid, the degree of orientation achieved by the method of the invention is directly proportional to the viscosity and inversely proportional to the pulse length and to the square of the saturation magnetization. Throughout the process of sequentially applying the magnetic field, the medium is allowed to harden and, hence, its viscosity is increased. As the orientation process continues, the freedom of motion of the crystals becomes more and more restrained so that at the end of the process the crystals are essentially frozen in a state such that subtsantially complete alignment of the crystal axes is obtained.

However, during this process of hardening of the medium, the yield stress of the medium increases. This increase in the yield stress acts as a detrimental factor in achieving a high degree of orientation since it tends to prevent the rotation of the particles to the desired directions. The use of short pulse lengths reduces the detrimental effects of the increase in the yield stress since it may be arranged that satisfactory orientation is obtained at a point in time before the increased yield stress becomes an important factor.

The application of a sequentially pulsed magnetic field along the easy axis of magnetization may be achieved in many ways, one of which is shown partially pictorially and partially schematically in FIG. 3. In that figure, a test tube 20 containing particles of a ferromagnetic material suspended within a medium 21 is positioned in the field of a coil 22 such that the direction of the magnetic field produced by a current through the coil lies along a direction corresponding to one of the body diagonals of the crystalline particles. The suspending medium may be a plastic, such as Hysol, well known to those in the art as being a class of liquid epoxy resins. The particles are single crystals with cubic crystal structure and arbitrary shape having a negative anisotropy constant. Coil 22 is arranged such that the direction of the magnetic field (as shown by dashed arrow 26) produced by a current through the coil lies at an angle of 54.4 with respect to the longitudinal direction of test tube 20. This relationship between the longitudinal axis of the test tube and the direction of the applied field assures that, as the tube is rotated about its longitudinal axis, the field is applied along the correct directions corresponding to the body diagonals. Coil 22 is connected to a source 23 of excitation pulses.

Initially, the coil is excited by a short pulse of current which applies a strong pulse of magnetic field to the crystal particles (corresponding, for example, to pulse 30 of FIG. 4). The application of this strong pulse of magnetic field tends to align one body diagonal of each of the crystal particles of the material parallel to this applied field.

Test tube 20 is connected to a source of rotation comprising a first gear 27 attached to test tube 20 and driven by a second gear 28 secured to a shaft 29 of a motor 35. Motor 35 is connected to a source of motor excitation voltage so that the motor shaft is continuously rotated at a constant rate. The rotation of shaft 29 causes ,the test tube to rotate continuously about its longitudinal axis in the direction of arrow 25.

When, during its first revolution, tst tube 20 is rotated to a position that is removed from its initial position, a second short pulse of magnetic field (corresponding, for example, to pulse 31 of FIG. 4) is applied. This second pulse tends to align a second body diagonal axis of each of the particles in a manner similar to that in which the first body diagonal was aligned. In a similar manner, when the test tube has reached positions which are and 270 removed from its initial position, third and fourth pulses of magnetic field (corresponding, for example, to pulses 32 and 33 of FIG. 4) are applied, respectively. These pulses of magnetic field tend to align the remaining two body diagonal axes of the particles. This process is cyclically repeated so that, as the test tube is continuously rotated, a short pulse is applied in turn along each of the directions corresponding to the relative directions of the body diagonals of the crystal particles.

The pulses are applied at times separated by precisely one fourth of the time required for a complete rotation of the container in order to give cyclic operation. In order to make the time of application of a pulse correspond to a well-defined orientation of the axes, it is necessary that the pulse length be small compared to the time between each of the pulses (i.e. the time for one fourth of a revolution). The optimum pulse length is dependent upon the viscosity of the suspending medium in such a way that the lower the viscosity of the medium which is used, the shorter the pulse duration.

The time between pulses is determined by the rate of revolution of the test tube. For a medium which is continuously hardening during the process, this time is chosen to allow a sufiicient number of pulses to be applied before the viscosity becomes very high. For a suspension medium having an initial viscosity on the order of 50-100 poises, for example, the pulse time duration is on the order of ten microseconds. The time between pulses may be chosen as twenty-five milliseconds so that the test tube is rotated at a rate of ten revolutions per second.

These figures are deemed to be representative of one particular embodiment of the method of the invention. Other pulse lengths and revolution rates may be chosen in accordance with the materials used. It is well to keep in mind the general principle that the larger the viscosity of the medium, the longer the pulse length and the slower the rate of revolution that is required.

It is believed that orientation of the material may be substantially completed after a few hundred revolutions of the test tube and, hence, after a few hundred applications of the pulses along each body diagonal axis. However, since the hardening of the medium usually takes longer than the orientation time, it is generally necessary to continue the application of the pulses until hardening occurs so as not to risk losing the orientation that has been achieved. After substantially complete orientation is obtained, it is possible to decrease the hardening time of the suspending medium by adding an appropriate accelerator. For example, if Hysol No. 6020 is used for the medium, it is possible to use Hysol Hardener C as the accelerator. As previously explained, the term Hysol represents a class of well known liquid epoxy resins. The particular Hysol resin and hardener suggested here provides a resin of medium viscosity and its general properties are well known. Both of these products are available commercially from the Houghton Laboratories, Inc, Olean, New York and are fully described by Technical .Data Bulletin ESS1 dated March 1958 and published by said company.

Alternatively, the material may be maintained in a fixed position and the coil may be moved in an appropriate manner so that the field produced by it is sequentially aligned with each of the appropriate axes. Any method which provides a relative motion between the material and the coils is suitable. Another alternative method would be to position a plurality of coils so that their magnetic fields lie along relative directions corresponding to the relative directions of the body diagonals of the material. The coils may then be excited sequentially by providing a switching device which supplies an excitation pulse to each of the coils in turn whereby a magnetic field pulse is cyclically applied along each of the required directions.

The above methods are not to be construed as the only embodiments of the method of the invention. For example, the suspension of the ferromagnetic particles need not necessarily be limited to suspension in a plastic medium and, indeed, need not necessarily be suspended in another material at all. As an alternative, the particles may be placed in a test tube and their freedom of motion may be controlled by a pressure device such as a piston inserted into the test tube. The particles in the test tube may then be ultrasonically vibrated so that the particles are given substantial freedom of motion equivalent to the freedom of motion enjoyed by the particles in the plastic fluid at the start of the previous described process. As the process continues and the pulses of magnetic field are sequentially applied, the vibration amplitude is gradually reduced and the pressure exerted by the piston on the particles is gradually increased so that the motion of the particles is gradually restrained during the process. This operation is equivalent to the restrained motion which occurs when the plastic medium gradually hardens in the previously described process. Thus, at the end of the process, the closely packed particles are correctly aligned along their easy axes of magnetization but, in this case, are not suspended in the plastic medium as before.

Although the process is generally carried out at room temperature, the invention is not limited in this respect since it may, in some cases, be desirable to perform the process at other temperatures depending upon the viscosity and rate of hardening of the plastic medium involved.

Although the process of the invention has been particularly described with respect to the orientation of crystals having a negative anisotropy constant, it will be obvious to those skilled in the art that the process is also applicable to the orientation of crystals having a positive an1sotropy constant. In that case, since the easy axes of magnetization lie along the directions of three cube edgm, the magnetic field is applied three times during each revolution. The pulses are cyclically applied at times separated by precisely one-third of a revolution and at the end of the process, the corresponding easy axes of magnetization of each of the crystals are correctly aligned.

Other methods may occur to those skilled in the art for obtaining the oriented particles without departing from the spirit and scope of this invention. Hence, the invention is not to be construed to be limited to the particular embodiments described herein except as defined by the appended claims.

What is claimed is:

1. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of positioning said ferromagnetic crystals having substantial freedom of motion in a non-magnetic volume to which a magnetic field may be applied, sequentially applying pulses of said magnetic field to said ferromagnetic crystals along directions c0rresponding to the relative directions of the crystallographic easy axes of magnetization of said ferromagnetic crystals to orientate a first crystallographic easy axis in the direction of said magnetic field and then orientating a second crystallographic easy axis With respect to said first axis and restraining said freedom of motion until said freedom of motion is stopped.

2. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of suspending said crystals having substantial freedom of motion in a non-magnetic medium, positioning said suspended particles in a volume to which a magnetic field may be applied, sequentially applying pulses of said magnetic field to said particles along directions corresponding to the relative directions of the crystallographic easy axes of magnetization of said crystals to orientate a first crystallographic easy axis in the direction of said magnetic field and then orientating a second crystallographic easy axis with respect to said first axis and restraining said freedom of motion until said freedom of motion is stopped.

3. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of suspending said crystals in a plastic medium that initially provides freedom of motion for said crystals, positioning said suspended particles in a volume to which a magnetic field may be applied, sequentially applying pulses of said magnetic field along selected directions corresponding to the relative directions of the crystallographic easy axes of magnetization of said crystals, and restraining said freedom of motion of said crystals as said magnetic field is sequentially applied until said freedom of motion is stopped.

4. A method of orienting ferromagnetic material composed of substantially cubic crystals having a plurality of predetermined given axes and having a negative anisotropy constant wherein the easy axes of magnetization correspond to the body diagonals of said crystals comprising the steps of suspending said crystals in a nonmagnetic plastic medium, positioning said suspended crystals in a volume to which a magnetic field may be applied, applying a first pulse of said magnetic field in a first direction corresponding to a first body diagonal of said crystals, applying a second pulse of said magnetic field along a second direction corresponding to a second body diagonal 0 said crystals, applying a third pulse along a third direction corresponding to a third body diagonal of said crystals, applying a fourth pulse along a fourth direction corresponding to a fourth body diagonal of said crystals, cyclically repeating the sequential application of said magnetic field along said first, second, third and fourth selected directions, and restraining the freedom of motion of said crystals as said pulses are cyclically and sequentially applied whereby the orientation of said crystals is maintained during said cycling process.

5. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of suspending said crystals in a non-magnetic medium that initially provides freedom of motion for said crystals, positioning said crystals in a volume to which a magnetic field may be applied, sequen tially applying pulses of said magnetic field along selected directions corresponding to the crystallographic easy axes of magnetization of said crystals, the duration of said pulses being approximately ten microseconds, and restraining the freedom of motion of said crystals as said magnetic field is sequentially applied until said freedom of motion is stopped.

6. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of suspending said crystals in a non-magnetic medium that initially provides freedom of motion for said crystals, positioning said crystals in a volume to which a magnetic field may be applied, sequentially applying pulses of said magnetic field along selected directions corresponding to the crystallographic easy axes of magnetization of said crystals, the duration of said pulses being approximately ten microseconds and the duration of time between said pulses being relatively longer, and restraining the freedom of motion of said crystals as said magnetic field is sequentially applied until said freedom of motion is stopped.

7. The method of orienting nickel ferrite crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of suspending said crystals in a liquid epoxy resin medium that initially provides freedom of motion for said crystals, positioning said suspended crystals in a volume to which a magnetic field may be applied, sequentially applying pulses of said magnetic field along selected directions corresponding to the crystallographic easy axes of magnetization of said crystals, said medium hardening so as to restrain the freedom of motion of said crystals as said magnetic field is sequentially applied until said freedom of motion is stopped, and adding an accelerator to said medium so as to increase the rate of hardening of said medium.

8. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of suspending said crystals in a non-magnetic medium that initially provides freedom of motion for said crystals, placing said suspended crystals in a container, positioning said container in a volume to which a magnetic field may be applied in a selected direction with respect to the longitudinal axis of said container, continuously rotating said container about its longitudinal axis at a constant rate, sequentially applying pulses of said magnetic field as said container is rotated at instants in time when said applied field direction coincides with the relative direction of the crystallographic easy axes of magnetization of said crystals, whereby said crystals tend to be aligned so that their easy axes of magnetization substantially parallel with each other, and restraining said freedom of motion as said pulses of magnetic field are applied until said freedom of motion is stopped.

9. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of suspending said crystals in a non-magnetic medium that initially provides freedom of motion for said crystals, placing said suspended crystals in a container, positioning said container in a volume to which a magnetic field may be applied at an angle of 544 with respect to the longitudinal axis of said container, continuously rotating said container about its longitudinal axis at a constant rate, sequentially applying pulses of said magnetic field as said container is rotated at instants in time when said applied field direction coincides with the relative directions of the crystallographic easy axes of magnetization of said crystals, whereby said crystals tend to be aligned so that their easy axes of magnetization are substantially parallel with each other, and gradually restraining said freedom of motion as said pulses of magnetic field are applied until said freedom of motion is stopped.

10. A method of orienting ferromagnetic crystals having a plurality of predetermined given crystallographic easy axes comprising the steps of placing said crystals in a container, positioning said container in a volume to which a magnetic field may be applied, vibrating said crystals so as to provide substantial freedom of motion of said crystals, sequentially applying pulses of magnetic field along selected directions corresponding to the crystallographic easy axes of magnetization of said crystals as said crystals are being vibrated, gradually reducing the amplitude of said vibration of said crystals, and gradually restraining the freedom of motion of said crystals as said magnetic field is sequentially applied until said freedom of motion is stopped and said crystals are maintained in their oriented state.

11. A method of orienting ferromagnetic material composed of substantially cubic crystals having a plurality of predetermined given axes and having a positive anisotropy constant wherein the easy axes of magnetization correspond to the sube edges of said crystals comprising the steps of suspending said crystals in a non-magnetic plastic medium, positioning said suspended crystals in a volume to which a magnetic field may be applied, applying a first pulse of said magnetic field in a first direction corresponding to a first cube edge of said crystals, applying a second pulse of said magnetic field along a second direction corresponding to a second cube edge of said crystals, applying a third pulse along a third direction corresponding to a third cube edge of said crystals, cyclically repeating the sequential application of said magnetic field along said first, second, and third selected directions, and gradually restraining the freedom of motion of said crystals as said pulses are cyclically and sequentially applied whereby the orientation of said crystals is maintained during said cycling process.

References Cited in the file of this patent UNITED ST TES PATENTS 2,532,876 Asche et al. Dec. 5, 1950 2,687,500 Jones et al Aug. 24, 1954 2,848,748 Crump Aug. 26, 1958 2,964,793 Blume Dec. 20, 1960 2,999,275 Blurne Sept. 12, 1961 OTHER REFERENCES Morrill: General Electric Review, August 1950, pages 16-21.

Watson et a1.: Journal of Applied Physics, V29, N13, pp. 306-308.

Popper: Journal of Electrical Engineers, V2, N20, pp. 450-457. 

1. A METHOD OF ORIENTING FERROMAGNETIC CRYSTALS HAVING A PLURALITY OF PREDETERMINED GIVEN CRYSTALLOGRAPHIC EASY AXES COMPRISING THE STEPS OF POSITIONING SAID FERROMAGNETIC CRYSTALS HAVING SUBSTANTIAL FREEDOM OF MOTION IN A NON-MAGNETIC VOLUME TO WHICH A MAGNETIC FIELD MAY BE APPLIED, SEQUENTIALLY APPLYING PULSES OF SAID MAGNETIC FIELD TO SAID FERROMAGNETIC CRYSTALS ALONG DIRECTION CORRESPONDING TO THE RELATIVE DIRECTIONS OF THE CRYSTALLOGRAPHIC EASY AXES OF MAGNETIZATION OF SAID FERROMAGNETIC CRYSTALS TO ORIENTATE A FIRST CRYSTALLOGRAPHIC EASY AXIX IN THE DIREC- 